1. Technical Field
The present disclosure relates to camera modules, in particular those manufactured using techniques known in the field of microelectronics.
2. Description of the Related Art
There are many applications for cameras were size is at a premium. Common examples of these are mobile phones and other mobile multimedia devices, though other applications also exist. To satisfy these demands, miniature camera modules are manufactured using microelectronics techniques.
Such modules often use sensors made using semiconductor technology, for example CMOS (complementary metal oxide semiconductor) or CCD (charge coupled device) technologies. These sensors come in the form of arrays of sensor cells on a piece of silicon, sometimes in association with some image processing circuitry. These sensors are very sensitive to light and to electromagnetic interference.
Stray light can saturate the sensor. Electromagnetic fields, when strong enough, can also disturb the functioning of the detectors cells. The result, in both cases, is the degradation, to a greater or lesser degree, of the image being generated by the sensor.
It is, therefore, desirable to protect the sensor and from electromagnetic fields and to ensure that the only light incident on the sensor is that which has passed via the optics.
Furthermore, in many situations, there is significant pressure to minimize the cost of the camera module.
FIG. 1 represents a cross-section of a miniature camera module 1 manufactured using known techniques.
A sensor die 2 has a lower surface to which are attached solder balls 3, for connections to a circuit board (not shown). On a top surface of the sensor die, there is arranged a sensor array 4 having attached to it micro-lenses 5.
In order to produce color sensors, a color filter mask (not shown) is placed in front of the sensor array 4. This mask contains areas of color filter placed at the same pitch as the individual sensor cells, such that each sensor cell has its own color filter in front of it. The color filters for neighboring cells are of a different primary color, for example red, green and blue. When the image is processed, the information from each of the groups of neighboring cells, typically four cells, is combined into information for a single pixel of the image. Thus the final image has ¼ the resolution of the actual sensor array.
In an area outside the sensor array, attached to the upper surface of the sensor die are small spacers 6. The small spacers are also attached to a lower surface of the glass plate, known as a cover class 7, such that the cover glass 7 is held parallel to and at a small distance above the sensor die 2. The purpose of the cover glass 7 is to protect the surface of the sensor array 4 from particles, particularly those generated during the assembly processing.
On an upper surface of the cover glass 7, there is attached a lower end of a spacer element 8. The spacer element 8 has a tubular or box cross-section and contacts the cover glass in an area outside sensor array.
To an upper end of the space element 8 there is attached an optical element 9, having focusing means, such as a lens assembly 10. The focusing means 10 is aligned with the sensor array.
To an upper surface of the optical element 9 is attached an aperture mask 11 forming an opening aligned with the focusing means 10 and the sensor array 4. The purpose of the aperture mask 11 is to prevent phantom images being created by rays of high incident angle, particularly those passing through the extremities of the focusing means 10.
The spacer element 8 has the purpose of placing the focusing means 10 on the optical element 9 at the correct operating distance from the sensor array 4. This distance is a function of a number of parameters such as the overall size of the sensor array 4, the density of cells in the sensor array 4 and the optical properties of the focusing means 10.
A shielding element 12 surrounds in the assembly of the sensor die 2, the cover glass 7, the spacer element 8, the optical element 9, and the aperture mask 11. The shielding element 12 is attached to the lower surface of the sensor die by a suitable means such as crimping and conductive glue 13. The top surface of the shielding element 12 has an aperture aligned with that of the aperture mask.
The purpose of the shielding element 12 is to prevent electromagnetic interference and to stop light entering the side of the camera module. In order to fulfill these functions effectively, it is both opaque and conductive, and is often made of a metal foil. In order to obtain an effective faraday cage, the shielding element 12 is connected to a ground connection via the conductive glue 13.
The presence of the shielding element increases the volume occupied by the camera module, particularly in the x-y directions. Furthermore it represents an additional cost in terms of both the material itself and the time-consuming assembly steps to put it in place. There may also be a yield loss associated with this part of the process.
The shielding element 12 represents an increase in weight. In order to mitigate this, it is possible to make the foil thin. However this has the disadvantage of making the shielding element fragile. Also, as described previously, the shielding element is attached by crimping and gluing. The joint is, to a certain extent, fragile.
This is of concern for the manipulation steps for assembly of the camera module 1 on to the printed circuit board, and significant care is taken to avoid yield loss at this step.
Traditionally, such camera modules have been assembled as individual units. For example, a spacer element 8 is placed on a sensor die 2+cover glass 7 sub-assembly and an optical element 9 added thereon. Were these steps to be performed collectively, a significant economy in manufacturing cost could be obtained.
The US patent application published as 2007/0052827 describes a method of coating the outside of a camera module. However there is no teaching of how to integrate this into an industrial assembly flow, or indeed much detail of such a flow. Nor is there any indication of the impact on the manipulation of the coated camera.
It is therefore desirable to provide a miniature camera module which is smaller and more robust. It is also desirable that the camera module be of lower cost in terms of materials and manufacturing, and be easier to assemble onto the printed circuit board of the equipment for which it is intended.